Fuel cells have been known for portable applications such as automotive and portable electronics technology for very many years, although it is only in recent years that fuel cells have become of serious practical consideration. In its simplest form, a fuel cell is an electrochemical energy conversion device that converts fuel and oxidant into reaction product(s), producing electricity and heat in the process. In one example of such a cell, hydrogen is used as fuel, air or oxygen as an oxidant and the product of the reaction is water. The gases are fed respectively into catalysing, diffusion-type electrodes separated by a solid or liquid electrolyte which carries electrically charged particles between the two electrodes. In an indirect or redox fuel cell, the oxidant (and/or fuel in some cases) is not reacted directly at the electrode but instead reacts with the reduced form (oxidized form for fuel) of a redox couple to oxidise it and this oxidised species is fed to the cathode.
An acknowledged problem concerning electrochemical fuel cells is that the theoretical potential of a given electrode reaction under defined conditions can be calculated but never completely attained. Imperfections in the system inevitably result in a loss of potential to some level below the theoretical potential attainable from any given reaction. Previous attempts to reduce such imperfections include the selection of catholyte additives which undergo oxidation-reduction reactions in the catholyte solution. For example, U.S. Pat. No. 3,294,588 discloses the use of quinones and dyes in this capacity. Another redox couple which has been tried is the vanadate/vanadyl couple, as disclosed in U.S. Pat. No. 3,279,949.
Several types of proton exchange membrane fuel cells exist. For example, in U.S. Pat. No. 4,396,687 a fuel cell is disclosed which comprises regenerable anolyte and catholyte solutions. The anolyte solution is one which is reduced from an oxidised state to a reduced state by exposure of the anolyte solution to hydrogen. According to U.S. Pat. No. 4,396,687, preferred anolyte solutions are tungstosilicic acid (H4SiW12O40) or tungstophosphoric acid (H3PW12O40) in the presence of a catalyst.
The preferred catholyte solution of U.S. Pat. No. 4,396,687 is one which is re-oxidised from a reduced state to an oxidized state by direct exposure of the catholyte solution to oxygen. The catholyte of U.S. Pat. No. 4,396,687 includes a mediator component comprising a solution of VOSO4. The mediator functions as an electron sink which is reduced from an oxidation state of V(V) to V(IV). The catholyte also includes a catalyst for regenerating the mediator to its oxidised state, (VO2)2SO4. The catalyst present in the catholyte of U.S. Pat. No. 4,396,687 is a polyoxometallate (POM) solution, namely H5PMo12V2O40.
Besides U.S. Pat. No. 4,396,687, a number of other attempts to use oxometallate catalysts have been made. For example, in U.S. Pat. No. 5,298,343, cathode systems comprising solid metal catalysts, oxometallates and metallic acids, such as molybdic acid are disclosed.
In addition, WO 96/31912 describes the use of embedded polyoxometallates in an electrical storage device. The redox nature of the polyoxometallate is employed in conjunction with carbon electrode material to temporarily store electrons.
The basic structure of the fuel cells used with the present invention is disclosed in EP1866997. The fuel cells disclosed herein comprise an anode and a cathode separated by an ion selective polymer electrolyte membrane, the cathode comprising a cathodic material and a proton-conducting polymeric material; means for supplying a fuel to the anode region of the cell; means for supplying an oxidant to the cathode region of the cell; means for providing an electrical circuit between the anode and the cathode; a non-volatile redox couple in solution in flowing fluid communication with the cathode, the redox couple being at least partially reduced at the cathode in operation of the cell, and at least partially re-generated by reaction with the oxidant after such reduction at the cathode. The examples disclose that the catholyte is maintained at 50° C.
EP1999811 discloses the redox fuel cells as used in the present invention, in combination with a polyoxometallate. This application discloses fuel cells that comprise an anode and a cathode separated by an ion selective polymer electrolyte membrane; means for supplying a fuel to the anode region of the cell; means for supplying an oxidant to the cathode region of the cell; means for providing an electrical circuit between the anode and the cathode; a non-volatile catholyte solution flowing in fluid communication with the cathode, the catholyte solution comprising a polyoxometallate redox couple being at least partially reduced at the cathode in operation of the cell, and at least partially re-generated by reaction with the oxidant after such reduction at the cathode, the catholyte solution comprising at least about 0.075M of the said polyoxometallate.
Zhizhina, E. G.; Simonova, M. V.; Odyakov, V. F.; Matveev, K. I. Applied Catalysis A: General 2007, 319, 91-97 discloses an increase in rate of redox potential of polyoxometallates at increased temperatures (100, 120, 130, 140, 150, 160° C.) under a PO2 of 4 atm for chemical synthesis applications. The paper then goes on to state that “increasing temperature, rate and depth of regeneration of any HPA by dioxygen highly grow”. However, the same paper also indicates that temperatures above 140° C. and preferably above 160° C. are beneficial. The aqueous polyoxometallate solutions disclosed herein are complex equilibrium systems containing different anions and cations.
Odyakov, V. F.; Zhizhina, E. G.; Maksimovskaya, R. I. Applied Catalysis A: General 2008, 342, 126-130 discloses that the regeneration of the polyoxometallate materials above requires “harder conditions” of 150-170° C. and PO2 of 2-4 atm. Zhizhina, E. G.; Odyakov, V. F. Applied Catalysis A: General 2009, 358, 254-258 describes a temperature of 150-160° C. being required for the re-oxidation of the polyoxometallate catalyst to proceed efficiently. Further, the regeneration conditions for the polyoxometallate catalyst used are a temperature of 170° C. and a PO2 of 4 atm.
Our own PCT/GB2013/051675 discloses the use of Na4H3PV4Mo8O40 and also of “acid POMs” (i.e. polyoxometallates containing no non-hydrogen cations) in fuel cells at elevated temperature and/or pressure.
Certain prior art fuel cells all suffer from one or more of the following disadvantages: they are inefficient; they are expensive and/or expensive to assemble; they use expensive and/or environmentally unfriendly materials; they yield inadequate and/or insufficiently maintainable current densities and/or cell potentials; they are too large in their construction; they produce unwanted by-products and/or pollutants and/or noxious materials; and they have not found practical, commercial utility in portable applications such as automotive and portable electronics. With respect to our prior disclosures of POM systems, we seek to improve cell performance and identify new classes of POM systems and optimal operating conditions with respect to them to provide enhanced fuel cell systems.